DESCRIPTION: (Applicant?s Description) The major goals of this project are to perform biochemical experiments at the single nucleosome or fiber level in buffer as visualized by scanning force microscopy. Experimental plans can be roughly separated into three parts: (1) Reversible compaction of individual fibers followed in buffer by SFM. Images of the same fiber in low, moderate and high salts will be used to generate models of fiber structure. A new, well-phased fiber will be created for this purpose. (2) What occurs when an elongating RNA polymerase encounters a nucleosome? Experiments are designed to ask what happens when a single RNA polymerase encounters a single nucleosome in buffer in real time. (3) How do nucleosomes in the Mouse Mammary Tumor Virus (MMTV) Long Terminal Repeat (LTR) interact with transcription factors? Are specific nucleosomes displaced or are their structures altered to allow transcription factor binding?